chore: add github SECURITY.md and CODEOWNERS

Author: divyamohan1993

.github/CODEOWNERS

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+# Default owners for everything in the repo.
+*       @divyamohan1993

.github/SECURITY.md

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+# Security Policy
+
+For the full security posture, threat model, and headers, see
+[`docs/SECURITY.md`](../docs/SECURITY.md).
+
+## Supported versions
+
+| Version | Supported |
+|---------|-----------|
+| 1.0.x   | yes       |
+| earlier | no        |
+
+## Reporting a vulnerability
+
+Please email **contact@dmj.one** with the words **SentinelCloud security**
+in the subject line. Do not open a public GitHub issue.
+
+We aim to acknowledge within 48 hours, triage within 7 days, and ship
+a fix within 30 days for high-severity reports.
+
+## Scope
+
+In scope:
+- The deployed Cloud Run service at `sentinelcloud.dmj.one`.
+- Anything published in this repository.
+
+Out of scope:
+- Brute force / volumetric DDoS against the public site.
+- Vulnerabilities in third-party services we depend on (Vertex AI,
+  Cloudflare, GitHub) unless they manifest as a config bug here.